Small blood vessels of the lactating mammary gland and of the lung in rats, mice and guinea-pigs were examined after feeding fat or after the intravenous injections of chyle or an artificial fat emulsion. In both tissues the vessels are lined by a continuous endothelium and basement membrane. By electron microscopy, chylomicra and particles of the artificial emulsion could be seen in the vessel lumen, but not in pinocytic vesicles and intercellular junctions, nor on the outer surface of the endothelium. The chylomicra and artificial particles were more numerous in vessels in the mammary gland than in the lung. In the mammary gland they were concentrated against the luminal surface of the endothelium and appeared to adhere closely to it. Both types of particle behaved similarly except that the artificial particles were seen rather more frequently within endothelial vacuoles or multivesicular bodies. In the lung, chylomicra and artificial particles were freely suspended in the blood plasma. The action of an enzyme, believed to be clearing factor lipase, on chylomicra and artificial fat particles in the mammary vessels could be demonstrated histochemically by light and electron microscopy. The possible relation of this enzyme to the close adhesion of fat particles to the endothelium and its role in the transport of fat through the vessel wall is discussed.